1. Field of the Invention
The invention relates to a process for preparing vinylchlorosilanes by non-catalyzed, thermal reaction of chlorosilanes with vinyl chloride using a ring-gap reactor having an, if appropriate, rotating displacement body.
2. Discussion of the Background
Vinyltrichlorosilane is a valuable intermediate, which, owing to its four reactive groups, is suitable for many applications. For example, it is used in the sizing of glass fibers and in the manufacture of cable materials.
DE-C 936 445, DE-A 22 10 189 and, in particular, DE-C 20 02 258 disclose that upon passing vinyl chloride/chlorosilane mixtures and especially vinyl chloride/trichlorosilane mixtures through appropriately heated empty tubes made of ceramic, glass or iron, industrially acceptable yields of vinylchlorosilanes are achieved. The reaction proceeds purely thermally, i.e., without a catalyst. In the reaction of trichlorosilane with vinyl chloride, the following reactions proceed.
Main reaction: EQU C.sub.2 H.sub.3 Cl+SiHCl.sub.3.revreaction.C.sub.2 H.sub.3 SiCl.sub.3 +HCl
In addition to the above equilibrium reaction, there are the following side reactions and secondary reactions: EQU C.sub.2 H.sub.3 Cl+SiHCl.sub.3.fwdarw.SiCl.sub.4 +C.sub.2 H.sub.4 EQU 4 SiHCl.sub.3.fwdarw.3 SiCl.sub.4 +Si+2 H.sub.2 EQU C.sub.2 H.sub.3 Cl.fwdarw.C.sub.2 H.sub.2 +HCl EQU C.sub.2 H.sub.2.fwdarw.2 C+H.sub.2 EQU C.sub.2 H.sub.4 +SiHCl.sub.3.fwdarw.C.sub.2 H.sub.5 SiCl.sub.3 EQU C.sub.2 H.sub.4 +SiHCl.sub.3.fwdarw.C.sub.2 H.sub.3 SiCl.sub.3 +H.sub.2 EQU C.sub.2 H.sub.3 SiCl.sub.3 +SiHCl.sub.3.fwdarw.Cl.sub.3 SiC.sub.2 H.sub.4 SiCl.sub.3
Despite these side and secondary reactions, according to DE 40 01 820 A1, the selectivity for vinyltrichlorosilane, based on the vinyl chloride used in deficient amounts, depending on the ratio of the starting materials and the degree of conversion, is from 50 to 98%, with tubes of from 122 to 150 cm in length and diameters of from 25 to 35 mm, residence times of from 0.2 to 20 seconds, reaction temperatures of from 400 to 750.degree. C. and pressures of from 1 to 3 bar. However, the output (or capacity) of the reactors in this case is only from 0.8 to 3.2 metric tons of vinyltrichlorosilane/month. Selectivity and degree of conversion are inversely proportional to one another; the reactor output passes through a maximum as a function of the degree of conversion. A high selectivity is thus accompanied by an unsatisfactory and economically unacceptable reactor output at a low degree of conversion. According to DE-A 20 02 258, although increasing the size of the tube diameter to 50 mm increases the reactor output in proportion to the greater reactor volume, with still greater diameters, the specific reactor output, based on the reactor volume, decreases. It is therefore impossible to increase the space-time yield of vinyltrichlorosilane, or even only maintain it, by increasing the diameter of the reactor tube to above 50 mm.
An improved process for preparing vinylchlorosilanes by reacting chlorosilanes with vinyl chloride is, according to DE 40 01 820 A1, carried out in a ring-gap reactor which has a treatable reaction tube having an internal diameter d.sub.1 in the interior of which is situated an, if appropriate, rotatable cylindrical displacement body which extends over the entire length of the reaction tube, is arranged axially symmetrically and has an outer diameter d.sub.2. In this case the relationship d.sub.1 =d.sub.2 +2a applies, where a is generally at least 1 cm and is always &lt;5 cm. If the displacement body is substantially shorter than the reaction tube, the yield is decreased. This finding corresponds to the teaching of the three abovementioned publications, according to which, in the case of empty tube reactors, the yield falls if the tube diameter exceeds 5 cm. According to DE 40 16 021 A1, the capacity of the reactor, or the space-time yield of the process, can be further increased using a ring-gap reactor if the reaction components are preheated to from 120 to 400.degree. C. prior to entry into the reactor. However, much reactor volume is still lost with this measure, in that the reaction components are heated to approximately 550.degree. C. in order to react further adiabatically in the remaining reactor volume.